Method of joining two members by friction stir welding

ABSTRACT

This invention provides a configuration of a joint that allows a satisfactory welded joint to be formed with reduced deformation of the joint region when two-face structures (panels) are friction-welded end to end. The panels  31, 32  each have two substantially parallel plates  33, 34  and a third member  35  connecting the two plates  33, 34 . The end portions of the plates  33, 34  of one panel  32  are friction-welded to the end portions of the plates  33, 34  of the other panel  32 . At least one of the panels has a plate  36  at its end for connecting the plates  33  and  34  and has a rigidity to support a pressing force produced during the friction welding.

This application is a Divisional application of application Ser. No.10/600,573, filed Jun. 23, 2003, which is a Divisional application ofapplication Ser. No. 08/820,231, filed Mar. 18, 1997, the contents ofwhich are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a friction stir welding method that isapplicable to panel welding used, for example, in aluminum alloy railwaycars and buildings.

A two-face structure (panel) for railway cars using hollow members isdisclosed in Japanese Patent Laid-Open No. 246863/1990, and anotherusing laminated panels, such as honeycomb panels, is disclosed inJapanese Patent Laid-Open No. 106661/1994.

The process of friction stir welding is performed by rotating a roundrod inserted in a joint region to heat and plasticize the joint regionthus forming a weld. This type of welding is generally applied to a buttjoint and a lap joint, and is described in WO 93/10935 (which is thesame as EP 061 5480B1 and the Japanese Announcement laid-openpublication No. Hei 7-505090 and in the publication Welding & MetalFabrication, January 1995, pp. 13-16.

SUMMARY OF THE INVENTION

In friction stir welding, the reaction of the plasticized metal beingextruded from immediately beneath the rotating tool (round rod) to thesurface during the welding results in a downward force acting on thejoint region. Thus, when this welding method is applied to a two-facestructure (panel), this downward force causes the joint material at thejoint region to flow downward, deforming the joint. This makes itimpossible to produce a satisfactory weld.

Two-face structures (panels) include hollow members made of extrudedaluminum alloy and honeycomb panels. Joining such panels has beenaccomplished by MIG welding and TIG welding. When friction stir weldingis applied to such a joint, the joint is bent down or the material inthe joint region is forced to flow down due to a downward force producedduring the friction stir welding.

The inventor has found the above phenomena in a variety of experiments.

It is a first object of this invention to provide a satisfactory weldedjoint by minimizing deformation of the joint region when two faces arefriction stir welded.

It is a second object of this invention to provide a satisfactory weldedjoint when one face is friction stir welded.

It is a third object of this invention to enable two faces to be weldedtogether in a short time with little deformation.

The first object can be achieved by placing at the joint region aconnecting member that joins two plates forming two faces.

The second object is realized by providing the members at the jointregion with a raised portion that protrudes toward the friction stirwelding tool side.

The third object is realized by disposing rotary tools for friction stirwelding on both sides of the objects to be welded, placing the rotationcenter of one of the tools on an extension of the rotation center of theother tool, and performing friction stir welding simultaneously.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical cross section of one embodiment of this invention.

FIG. 2 is a vertical cross section of FIG. 1 after friction stirwelding.

FIG. 3 is a vertical cross section of another embodiment of thisinvention.

FIG. 4 is a vertical cross section of FIG. 3 after friction stirwelding.

FIG. 5 is a vertical cross section of another embodiment of thisinvention.

FIG. 6 is a vertical cross section of FIG. 5 after friction stirwelding.

FIG. 7 is a vertical cross section of still another embodiment of thisinvention.

FIG. 8 is a vertical cross section of FIG. 7 after friction stirwelding.

FIG. 9(A) to 9(D) are vertical cross sections showing steps in theprocedure of friction stir welding of a further embodiment of thisinvention.

FIG. 10 is a vertical cross section of a further embodiment of thisinvention.

FIG. 11 is a vertical cross section of a further embodiment of thisinvention.

FIG. 12 is a vertical cross section of a further embodiment of thisinvention.

FIG. 13 is a vertical cross section of a further embodiment of thisinvention.

FIG. 14 is a vertical cross section of a further embodiment of thisinvention.

FIG. 15 is a perspective view of a structural body of a railway car.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiment shown in FIG. 1 has a joint configuration of the abuttingtype between hollow members 31, 32 which are in the form of panels. Thehollow members 31, 32 have vertical plates 36, 36 at their ends in thewidth direction. Before the welding, the vertical plates 36, 36 aredisposed immediately beneath a rotary tool 50. The vertical plates 36,36 are opposed to and in contact with each other. If they are spacedapart, the distance is small and approximately 1 mm. On the extension ofthe interface between the vertical plates 36, 36 lies the center of aprojection 52. The vertical plates 36, 36 have a stiffness which issufficiently strong to sustain the downward force mentioned earlier. Thevertical plates 36 are perpendicular to two plates 33, 34 in each panel.The hollow members 31, 32 are formed by extruding an aluminum alloy. Theupper and lower faces of the hollow member 31 are flush with thecorresponding upper and lower faces of the hollow member 32. That is,the hollow members 31, 32 have the same thickness. This is true also ofthe succeeding embodiments. During the friction stir welding process,the boundary 53 between a large-diameter portion 51 and the projection52 of a small-diameter of the rotary tool 50 is situated above the uppersurfaces of the hollow members 31, 32. Numeral 35 designates a pluralityof members that are arranged in the form of trusses to connect the twoplates 33 and 34. The hollow members 31, 32 each have bilaterallysymmetrical end portions and are mounted on a bed (not shown) where theyare fixed immovably. The bed also lies under the vertical plates 36, 36.

The friction stir welding process is performed by rotating the tool 50,plunging the projection 52 into the joint region of the hollow members31, 32, and moving the projection 52 along the joint region. Therotating center of the projection 52 is located between the two verticalplates 36, 36.

FIG. 2 shows the two panels after they have been friction stir welded.Reference number 45 denotes the shape of a weld bead after welding. Onthe extension of the border line between the vertical plates 36, 36 thewidth center of the weld bead 45 is situated. The bead 45 lies in anarea on the extension of the thickness of the vertical plates 36, 36.The depth of the weld bead 45 is determined by the height of theprojection 52 at the lower end of the rotary tool 50 inserted in thejoint region.

With this construction, because the vertical plates 36, 36 perpendicularto the plates 33, 34 sustain the vertical force produced during thefriction stir welding, the joint region does not bend, offering asatisfactory joint as shown in FIG. 2. The vertical plate 36 is madeperpendicular to the plates 33, 34 in each panel as much as possible.

The vertical plate 36 may be perforated to achieve a lighter weight.This is true also of the succeeding embodiments.

Welding of the lower side is carried out by turning the hollow panelmembers upside down after the welding of the one side is completed.

The embodiment of FIG. 3 has a vertical plate 36 at the end of onehollow member 31, but not at the opposing end of the other hollow member32. Corners in the vertical direction of the vertical plate 36 of thehollow member 31 are recessed so as to receive the ends of projectingpieces 38, 38 of the hollow member 32. These recessed portions are openin a direction of the thickness of the hollow member 31 and in adirection perpendicular to the thickness direction (toward the hollowmember 32 side). When the projecting pieces 38 are placed (superposed)on the recessed portions, there is actually a clearance between themalthough they are in contact with each other in the figure. There isalso a gap between the front ends of these members (i.e., between theprojecting pieces 38, 38 and the corners 33 a, 34 b). The abutting jointportions on the upper face side of the two hollow members 31, 32 and thevertical plate 36 are situated directly below the center of the rotarytool 50. The rotating center of the projection 52 of the welding tool 50is disposed on an extension of the center line of the thickness of thevertical plate 36. That is, the joint region of the plate 33 (34) andplate 33 (34) is situated on the extension of the center line of thethickness of the vertical plate 36. The corners 33 b, 34 b extendingfrom the plates 33, 34 to the recessed portions lie on an extension ofthe center line of the thickness of the vertical plate 36. Consideringthe gap between the corners 33 b, 34 b and the projecting pieces 38, thecorners 33 b, 34 b are situated slightly to the left of the extension ofthe center line of the thickness of the vertical plate 36. The verticalplate 36 has a rigidity to support the downward force. The horizontalgap between the front ends of the projecting pieces 38 and the hollowmember 31 is similar to that shown in FIG. 1. The height of theprojection 52 of the welding tool 50 is approximately equal to thethickness of the projecting piece 38. The region that is plastic andfluid extends below the projecting piece 38, and comes to have an arealarger than the diameter of the projection 52, and the two hollowmembers 31, 32 are friction stir welded. It is desirable that thefriction stir weld be so formed as to extend beyond the contact areabetween the underside of the projecting piece 38 and the vertical plate36.

FIG. 4 shows the state of the joint after being welded. The weld bead 45is formed such that the width center of the weld bead 45 is situated onan extension of the thickness center of the vertical plate 36.

To support the vertical force, it is desirable for the rotating centerof the tool 50 to be located on the extension of the center line of thethickness of the vertical plate 36. To make the quantity of the joint ofthe left and right hollow members 31, 32 equal, it is desirable for thecorners 33 b, 34 b to be situated on the extension of the thicknesscenter line of the vertical plate 36. While the projection 52 of thetool 50 should preferably be placed in the range between extension linesof the thickness of the vertical plate 36, the thickness of the verticalplate 36 is determined by the vertical force, the position of theprojection 52 and the strength of the vertical plate 36. Hence, theremay be a case in which the thickness of the plate 36 is smaller than thediameter of the projection 52. In view of the possible errors of theposition of the rotary tool 50 and those of the corners 33 b, 34 b, itis desirable for the corners 33 b, 34 b to be positioned in the rangebetween extension lines of the thickness of the vertical plate 36, andat least a part of the projection 52 of the tool 50 to be situated inthis range. This arrangement enables the vertical plate 36 to receive atleast a part of the vertical force, substantially preventing thedeformation of the joint. As a result, a satisfactory joint can beformed. When the bead 45 is taken as a reference, although the bead 45is slightly larger than the projection 52, the same as above can besaid. This is true also of the other embodiments.

Compared with the case of FIG. 1, this joint configuration can minimizea sinking in the surface of the joint region even when the horizontalgap between the projecting piece 38 and the hollow member 31 is large.As a result, the joint has a good appearance and requires a reducedamount of putty for painting. This is because the gap between the twomembers is terminated at a depth equal to the thickness of theprojecting piece 38. It is also considered that this joint configurationcan reduce the weight. Further, because one of the hollow members isfitted into the other, the positioning in the height direction of thetwo members can be accomplished easily.

The ends of the hollow member 31 are bilaterally symmetrical in shape.The ends of hollow member 32 are also bilaterally symmetrical.Alternatively, the hollow member 31 may have one end shaped as shown inFIG. 3 and the other end shaped like the end of the hollow member 32 ofFIG. 3.

In the embodiment of FIG. 5, there is virtually no vertical plate 36immediately below the corners 33 b, 34 b of the recessed portion of thehollow member 31. The right end of the vertical plate 36 lies on theextension of the corners 33 b, 34 b. On this extension the rotatingcenter of the tool 50 is located. The end portion of the hollow member31 is given a rigidity to sustain the vertical force by making the lowerprojecting piece 37 at the joint thicker and increasing the size of thearcs extending from the front ends of the projecting pieces 37 to theplate 36. The projecting pieces 38 of the other hollow member 32 arereceived in the recessed portions of the projecting pieces 37, as in thepreceding embodiment of FIG. 3. The second hollow member 32 has avertical plate 36 near the projections 38 for connecting the upper andlower plates 33, 34. This arrangement prevents the joint region frombeing defective even when there is no vertical plate 36 directly belowthe corners of the recessed portions. It is noted, however, that belowthe range of the bead 45 there is a vertical plate 36 of the panel 31.FIG. 6 shows the state after welding.

In the embodiment of FIG. 5, the plate 36 of the hollow member 32 may beremoved.

FIG. 7 shows another embodiment, representing a variation of thepreceding embodiment of FIG. 5, in which the joint region of the twohollow members 31, 32 is provided with raised portions 37 a, 38 aprotruding outside. This makes the joint region thick. The heights ofthe raised portions 37 a, 38 a are equal. Other parts are similar tothose of FIG. 5, except that the vertical plate 36 and the projections37 are slightly thinner.

With this configuration, if there is a gap between the raised portions37 a and 38 a before welding, the gap is filled with the material of theraised portions 37 a, 38 a, when welded, improving the appearance andreducing the amount of putty required.

In conventional welded joints, the weld bead has a sunken portion orrecess corresponding to the volume of the lost material 41 that hasflowed down due to the downward force. In the joint configuration ofFIG. 7, the rotary tool 50 plasticizes the raised portions 37 a, 38 aand forces them downward making up for the lost volume of the material41. Thus, formation of recess can be prevented, providing a satisfactorywelded joint. FIG. 8 shows the shape of bead 45 after welding. Afterwelding, unnecessary parts, if any, are cut off as shown.

The raised portions 37 a, 38 a can also be applied to the embodiments ofFIG. 1, 3 and 5 and to subsequent embodiments.

FIGS. 9(A)-9(D) show a further embodiment, which allows welding at theupper and lower faces from only one side. The ends of the hollow members31, 32 on the lower side having projecting pieces 34 a, 34 a protrudingflush with the lower plates 34, 34 substantially toward the opposinghollow member sides. The front ends of the projecting pieces 34 a, 34 aare virtually in contact with each other. The front ends of the upperplates 33, 33 are offset back from the front ends of the lower plates 34a, 34 a. The front ends of the upper plates 33, 33 are connected to thelower plates 34, 34 through the vertical plates 36, 36. The verticalplates 36, 36 are connected to intermediate portions of the lower plates34. The top portions of the vertical plates 36, 36 are provided withrecessed portions 39, 39 that receive a joint 60. When mounted on therecessed portions 39, 39, the upper surface of the joint 60 is flushwith the upper face of the upper plates 33, 33. The distance between thetwo vertical plates 36, 36 is long enough for the rotary tool 50 to beinserted and is as short as possible. The relation between the verticalplates 36 and the recessed portions 39 is the same as explained withreference to the embodiments of FIG. 5 and FIG. 7.

The welding procedure will be described below. In the state of FIG.9(A), the abutting ends of the lower plates 34 a, 34 a are welded by therotary tool 50. At this time, the hollow members 31, 32, including thejoint region of the plates 34 a, 34 a, are mounted on a bed. The uppersurface of the bed (that backs the bead) is flat. The height of theprojection 52 of the rotary tool 50 is smaller than the thickness of theplates 34 a, 34 a. This design ensures that the bottom surface afterwelding is flat. Thus, the bottom side can easily be used as an outersurface of the structure of a railway car or a building (the outersurface being the surface on which no decorative plate is mounted).Generally, the upper face of the friction stir welded joint tends to beuneven (at a boundary portion 51).

Next, as shown in FIG. 9(B), the joint 60 is mounted between the twohollow shape members 31, 32. The joint 60 in the illustrated example isT-shaped in vertical cross section. When both ends of the joint 60 areplaced on the recessed portions 39, 39, the lower end of a verticalportion 61 has a clearance between it and the weld bead on the lowerplate. The vertical portion 61 may be omitted.

Next, as shown in FIG. 9(C), the joint portion between the joint 60 andthe hollow member 31 is friction stir welded by the rotary tool 50. Therotary tool 50 need not be the same as used for the weld illustrated inFIG. 9(A).

Then, as shown in FIG. 9(D), the joint portion between the joint 60 andthe hollow member 32 is friction stir welded by the rotary tool 50.

This procedure allows the welding to be performed from one side andeliminates the need for inversion of the panels. With the inversion ofthe panels eliminated, there is an advantage that the time required forinversion and positioning and need for an inversion device areunnecessary, and even the assembly precision is improved.

FIG. 10 shows another embodiment, in which both the upper and lowersides of the hollow members 51, 52 are friction stir welded at the sametime. A rotary tool 50 a for the lower side is disposed vertically belowthe welding tool 50 for the upper side. The projection 52 of the secondwelding tool 50 a faces up. The two welding tools 50, 50 a facing eachother are moved at the same speed to perform friction stir welding.Denoted by 70, 70 are beds (tables). The rotating centers of the tools50 and 50 a are on the same line on which the joint region of the hollowshape members 31, 32 is located.

Because with this arrangement the rotating center of the second tool 50a is positioned on the extension of the rotating center of the firsttool 50, the forces applied to the panels balance with each otherallowing the joint to be welded in a short time with little deformation.Further, because there is no need to invert the hollow members 31, 32,the welding can be performed in a short time with little deformation ofthe joint.

This procedure can be applied to other embodiments.

The preceding embodiments have used hollow members as panels to bejoined. The following embodiments show friction stir welding as appliedto honeycomb panels. As shown in FIG. 11, the honeycomb panels 80 a, 80b comprise two surface plates 81, 82, core members 83 havinghoneycomb-like cells, and edge members 84 arranged along the edges ofthe surface plates 81, 82, with the core members 83 and the edge members84 soldered to the surface plates 81, 82 to form integral structures.The surface plates 81, 82, the core members 83 and the edge members 84are made of aluminum alloy. The edge members 84 are made by extrusionand have a rectangular cross section. All sides of this rectangularcross section are greater in thickness than the surface plates 81, 82.The vertical sides of the mutually contacting edge members 84, 84 havethe same thickness as shown in FIG. 1. The two honeycomb panels 80 a, 80b have the same thickness.

The welding procedure in the embodiment of FIG. 11 corresponds to theone shown in FIG. 1. The height of the projection 52 of the rotary tool50 is larger than the thickness of the face plates 81, 82. This allowsthe face plates 81, 82 and the edge members 84, 84 to be welded. Theload acting on the panels 80 a, 80 b is transmitted mainly by the edgemembers 84. After being fabricated, the panels 80 a, 80 b are assembledand friction stir welded.

The embodiment of FIG. 12 corresponds to the one shown in FIG. 3. Theedge member 84 of the honeycomb panel 80 a has a generally rectangularcross section and has recesses at the corners. The edge member 84 of thehoneycomb panel 80 b is like a channel, with its opening facing thehoneycomb panel 80 a. The open ends of the edge member 84 are mounted onthe recessed portions of the edge member 84 of the honeycomb panel 80 a.

The honeycomb panel corresponding to FIG. 5 can be fabricated in asimilar manner.

FIG. 13 shows still another embodiment that corresponds to FIG. 7. Aftertwo honeycomb panels 80 a, 80 b are assembled, a plate 86 is placed onthe face plates 81, 81 and temporarily welded to them. The plate 86makes up for the material that is plasticized and flows out. In FIG. 12,one vertical piece of the edge member of the honeycomb panel 80 a isremoved. The vertical force is supported by the thickness of thehorizontal piece of the edge member 84 and the surrounding parts.

FIG. 14 shows a further embodiment of this invention. The precedingembodiments up to FIG. 13 include panels having two faces (face plates),whereas the embodiment of FIG. 14 includes panels 91, 92 havingvirtually a single face (face plates 94, 94). Friction stir welding isperformed at two locations, at the abutting ends of the panels 91, 92,the outside with face plates 94 and the inner side with no face plates.Therefore, the joint regions on the inner side are provided with narrowface plates (face plates 93, 93). The narrow face plates 93, 93 aresupported by vertical plates 96, 96. In this example, too, the verticalplates 96 are virtually perpendicular to the face plates 93,94. The faceplates 93, 94 are provided with raised portions 37 a, 38 a similar tothe ones shown in FIG. 7. The face plates 94, 94 have a plurality ofreinforcing ribs (plates) 95, 95 at specified intervals. The ribs 95 areT-shaped in cross section. The top surfaces of the ribs 95 are flushwith those of the face plates 93 of the joint region. To the topsurfaces reinforcing members (such as pillars) may be welded, or the topsurfaces serve as mounting seats for articles. Further, the face plates93, 93 also serve as a seat for controlling the height of the tool 50. Amovable body carrying the tool 50 travels along the face plates 93, 93.Because of the provision of the face plates 93, 94, the panels 91, 92can also be said to form a two-face structure. The panels 91, 92 areextruded shape members.

While FIG. 14 shows the vertical plates 96, 96 of the panels 91 and 92opposing each other at the joint region, as in the configuration of FIG.1, it is possible to place one of the vertical plates over the other, asshown in FIG. 3, 5 and 7.

FIG. 15 shows an example of application of this invention to thestructural body of a railway car. The structural body has side bodies101, a roof body 102, a floor body 103, and gable bodies 104 at the endsin the longitudinal direction. The side bodies 101 and the roof body 102have panels 31, 32, 80 a, 80 b, 91, 92 whose long sides are oriented inthe longitudinal direction of the car. The joint between the side bodies101 and the roof body 102 and between the side bodies 101 and the floorbody 103 is accomplished by MIG welding. The roof body 102 and the sidebodies 101 are often shaped into arcs in cross section. When the panels91, 92 are used for the side bodies 101, the side having the verticalplates 96 and ribs 96 is made to face to the interior of the car and thereinforcing members constitute pillars.

The panels 31, 32 of FIG. 9 may be combined in a mirror-imagearrangement. The end of the projecting plate 34 a of each panel isplaced on the recessed portion 39 of the plate 33 of the other panel.This obviates the use of the joint 60 and allows for simultaneousfriction stir welding of the joint region both from above and below. Theplates 33, 34 a can be provided with raised portions, as shown in FIG.7.

1. A friction stir welding method of a panel to a plate member, wherein:said panel comprises a first plate, a second plate substantiallyparallel to said first plate, plural ribs for connecting said firstplate and said second plate, and a vertical plate for connecting an endportion in a width direction of said first plate and said second plateand being substantially perpendicular to said first plate and saidsecond plate; and a connection portion between said first plate of saidpanel and said vertical plate, providing a recessed portion which opensin a thickness direction of said first plate of said panel and in adirection perpendicular to said thickness direction; arranging an endportion of said plate member in said recessed portion; and friction stirwelding an overlapped portion of said end portion of said plate memberand said recessed portion of said panel from an outer side of anextension line of a plate thickness of said vertical plate of saidpanel.
 2. A friction stir welding method according to claim 1, wherein afriction stir welded bead formed by said friction stir welding, lies inat least a range of a plate thickness of said vertical plate of saidpanel.
 3. A friction stir welding method according to claim 1, wherein acenter, in a width direction, of a friction stir welded bead formed bysaid friction stir welding, lies in a range of a plate thickness of saidvertical plate of said panel.
 4. A friction stir welding method of afirst panel to a second panel, wherein: said first panel comprises afirst plate, a second plate substantially parallel to said first plate,plural ribs for connecting said first plate and said second plate, and avertical plate for connecting an end portion in a width direction ofsaid first plate and said second plate and being substantiallyperpendicular to said first plate and said second plate; and aconnection portion between said first plate of said first panel and saidvertical plate, providing a recessed portion which opens in a thicknessdirection of said first plate of said first panel and in a directionperpendicular to said thickness direction; said second panel comprises afirst plate, a second plate substantially parallel to said first plateof the second panel, and plural ribs for connecting said first plate ofsaid second panel and said second plate of said second panel; arrangingan end portion of said first plate of said second panel in said recessedportion of said first panel; and friction stir welding an overlappedportion of said end portion of said first plate of said second panel andsaid recessed portion of said first panel from an outer side of anextension line of a plate thickness of said vertical plate of said firstpanel.
 5. A friction stir welding method according to claim 4, wherein afriction stir welded bead formed by said friction stir welding, lies atleast in a range of a plate thickness of said vertical plate of saidfirst panel.
 6. A friction stir welding method of a first panel to asecond panel, wherein: said first panel comprises a first plate, asecond plate substantially parallel to said first plate, plural ribs forconnecting said first plate and said second plate, and a vertical platefor connecting an end portion in a width direction of said first plateand said second plate and being substantially perpendicular to saidfirst plate and said second plate; a connection portion between saidfirst plate of said first panel and said vertical plate, providing arecessed portion which opens in a thickness direction of said firstplate of said first panel and in a direction perpendicular to saidthickness direction; a connection portion between said second plate ofsaid first panel and said vertical plate, providing another recessedportion which opens in a thickness direction of said second plate ofsaid first panel and in a direction perpendicular to said thicknessdirection of said second plate; said second panel comprises a firstplate, a second plate substantially parallel to said first plate of thesecond panel, and plural ribs for connecting said first plate of thesecond panel and said second plate of said second panel; arranging anend portion of said first plate of said second panel in said recessedportion of said first panel; arranging an end portion of said secondplate of said second panel in said another recessed portion of saidfirst panel; first friction stir welding an overlapped portion of saidend portion of said first plate of said second panel and said recessedportion of said first panel from an outer side of an extension line of aplate thickness of said vertical plate of said first panel; and secondfriction stir welding an overlapped portion of said end portion of saidsecond plate of said second panel and said another recessed portion ofsaid first panel from an outer side of an extension line of said platethickness of said vertical plate of said first panel.
 7. A friction stirwelding method according to claim 6, wherein: a friction stir weldedbead, formed by said first friction stir welding, at said end portion ofsaid first plate of said first panel, lies at least in a range of platethickness of said vertical plate of said first panel; and a frictionstir welded bead, formed by said second friction stir welding, at saidend portion of said second plate of said first panel, lies at least in arange of said plate thickness of said vertical plate of said firstpanel.
 8. A friction stir welding method according to claim 7, wherein arespective center in a width direction of both of the friction stirwelded beads lies in a range of said plate thickness of said verticalplate of said first panel.